A6-3

The 16th IAIN World Congress 2018 in Makuhari Messe, Chiba, Japan in November 2018**Peer Reviewed Paper**Performance Evaluation of CLAS and PPP Using
Correction Data via QZSSSo Takahashi, Haoyan Zhang, Nobuaki Kubo
Graduate School of Marine Science and Technology, Course of Maritime Technology and Logistics
Tokyo University of Marine Science and Technology
Tokyo, Japan
so.takahashi.row@gmail.com, zhysky333@gmail.com, nkubo@kaiyodai.ac.jp

Abstract—The need for high-accuracy positioning (centimeterlevel) has been increasing, particularly in self-driving cars,
agriculture, and construction. In 2017, Japan launched a QuasiZenith Satellite System (QZSS) comprising three satellites
successfully leading to a constellation of four satellites tostrengthen the positioning services. The QZSS, which isscheduled to commence its operations in November 2018, is
expected to implement three reinforcement services: centimeterlevel augmentation service (CLAS), precise point positioning(PPP), and sub-meter level augmentation service (SLAS). These
new services would access the correction data via QZSS. In thisstudy, we evaluated the efficacy of these systems by way of
conducting experiments on static and kinematic positioning,whereby we used two new GNSS receivers to decode the L6
signals broadcast from the QZSS. We used the RTK Fix rate,
standard deviation, and statistical percentile as the tools forevaluation. The experiments on static positioning using CLAS
showed a positioning accuracy within 10 cm in both horizontal
and vertical (height) directions. For kinematic experiments of
CLAS, open-sky condition was applied. The overall results of the
PPP experiments were similar to the previous research studies
for both static and kinematic conditions, while the accuracy was
up to 10 cm achieved after a convergence period of 15-20 min
approximately. The experimental results of SLAS showed an
improved accuracy for the static condition compared with astand-alone system. We propose to continue the studies acrossJapan over a period of one year to confirm the sustained
consistency of these correction services.
Keywords—Quasi-Zenith Satellite System; Centimeter Level
Augmentation Service; Precise Point Positioning; Sub-meter Level
Augmentation Service

I. RESEARCH BACKGROUND AND OBJECTIVE
In September 2010, the first Japanese Quasi-Zenith Satellite
System (QZSS) was launched by the Japan Aerospace
Exploration Agency (JAXA). The positioning service is foundto be stable as recognized by the public through the academic
societies in Asia. The new units (2, 3, and 4) of QZSS launchedin 2017 are scheduled to commence the full-scale positioning
service with effect from November 2018. In contrast to thepresent positioning services of QZSS that are confined only tothe complimentary functions concerning GPS, the users are
expected to have an enhanced access to three new services
from QZSS: centimeter level positioning service (CLAS) and
precise point positioning (MADOCA-PPP) in the L6 signal
band, and sub-meter positioning service (SLAS) in the L1S
signal band. These are positioning services of first kind not
only to Japan but to the entire world, which would be achieved
with the help of approximately 1300 electronic GNSS

reference points of Geospatial Information Authority of Japan.
The new services of QZSS are as shown below (Fig. 1).

Fig. 1. New Services of QZSSCLAS is an innovative system which employs the RTK
positioning (PPP-RTK) through a single station and a one way
communication from satellites, whereby the correction
information from the base stations are captured at the roverside via QZSS using its L6 signal band.
On the other hand, PPP continues to receive the test signals
from QZSS, and its performance has been verified by several
research institutes and companies over the past few years. L6
signal band comprises two channels: one for the correction
information for CLAS (D1 channel) and the second one for
precise ephemeris and clock information (D2 channel) for PPP.
Both the CLAS and the PPP exhibit three advantages: first,
there is no limitation on the baseline length; second, it ispossible to perform high-accuracy positioning even in areas
devoid of internet; finally, there is no need for a contract toobtain the correction information.
SLAS too has the same reinforcement function like CLAS,
whereby pseudo-range correction information is superimposedon L1S signal band. It is a system that realizes the DGNSS
positioning with single station and one way communicationfrom satellites. It is expected to apply even to the pedestrians,
cyclers, and ship because the position can be determined at
sub-meter level on a terminal which can receive and decode the
L1S signals.
In various industrial applications, there is a critical need forthe positioning information with a centimeter level accuracy.
Introduction and testing are most progressive in the Japanese
agricultural field from the viewpoint of utilization of precise
positioning. In 2017, Quasi-Zenith Satellite System Services

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Inc., Hokkaido University in collaboration with threemanufacturers of agricultural machinery succeeded to operate
self-driving agricultural tractors using the positioning results of
CLAS. Being a reinforcement function of QZSS, the CLAS isalso expected to support self-driven cars owing to its
compatibility with automatic systems. Thus, new products andproposals are expected to boom following the official
commencement of QZSS operations.
In this study, we evaluated the positioning accuracy of
CLAS, PPP, and SLAS based on the expansion of the QZSS
services mentioned above. We present in this paper theexperimental results of the static and kinematic positioning
using CLAS and PPP. For SLAS, we present the static
positioning accuracy of DGNSS positioning by baseline length
using electronic reference points. We investigated thecompliance of each of these reinforcement services to thespecifications published by the cabinet office in Japan. In thisregard, it is necessary to evaluate the positioning accuracy inopen-sky condition, especially for CLAS; however,considering that the obstacles such as mountains andoverpasses are practically inevitable in a real-time situation, weinvestigated the effect of overpasses on kinematic positioning.
This research is also aimed at evaluating the effectiveness ofthese reinforcement services for automatic driving. The results
helped establish that CLAS can be applied to the running cars
if an accurate positioning, for example, within 10 cm, isachievable.
II.

BRIEF DESCRIPTIONS OF CLAS, PPP AND SLAS

A. Centimeter Level Augmentation Service (CLAS)
To carry out highly precise satellite positioning, the datafrom the Geospatial Information Authority of Japan’s GNSSbased control stations called Japanese GEONET are gathered
and used. Approximately 300 base stations are used. An imageof Japanese GEONET is as shown below (Fig. 2). Correctionused in CLAS is transmitted by QZSS L6 signals are nottransmitted by GPS, hence dedicated receivers are required.
The concept of CLAS is PPP-RTK positioning, and the concept
image of CLAS is also as shown below (Fig. 3)

Fig. 3. Concept Image of CLAS

It is expected that this service will be used for surveying,intelligent construction (construction methods in whichconstruction machinery is operated with high precision), and eagriculture (methods for agricultural land management inwhich agricultural machinery is operated with high precision
automatically) mentioned in Section I. This service can be usedon L6-signals receivers.
B. Precise Point Positioning (MADOCA-PPP)
PPP is also a technique to calculate accurate user positions
using precise orbit and clock of GNSS without any reference
stations. JAXA had been conducting real-time PPP
experiments using L6 signals from QZSS. Moreover, JAXA
had developed Multi-GNSS orbit and clock estimator called“MADOCA (Multi-GNSS Advanced Demonstration tool for
Orbit and Clock Analysis)” for PPP users in Asia and Oceania
regions. The concept image of MADOCA-PPP and sometarget applications are as shown below (Fig. 4 and Fig. 5).
MADOCA-PPP is available in offshore and in some areas
where infrastructures are undeveloped (buoys for tsunami,
offshore platforms, mining areas and so on), thus it is expectedthat applications utilizing the characteristics is utilized in suchareas. The difference between CLAS and PPP is theconvergence time. The convergence time of CLAS is about 1
minutes and the convergence time of PPP is about 15-30
minutes.

Fig. 4. Concept Image of MADOCA-PPP

Fig. 2. Japanese GEONET (Approximately 1300 Points)

Fig. 5. Target Applications of MADOCA-PPP

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C. Sub-meter Level Augmentation Service (SLAS)
To reduce errors in satellite positioning, information thatcan be utilized to decrease these errors (sub-meter levelaugmentation information) – such as ionospheric delay, orbit
errors and clock errors – is transmitted by QZSS. The big
difference between CLAS/PPP and SLAS is what type ofobservation is used in the positioning. As for SLAS, pseudorange is used in the positioning, therefore, the accuracy is not
cm level but sub-meter level.
Normally ionospheric delay errors can be solved by using
dual-frequency receivers. However, these receivers are
generally large and expensive, hence it is expected thatexisting single-frequency receivers can be improved and used
for this purpose. L1S signals that transmit sub-meter levelaugmentation information is the same type of signal as the
L1C/A positioning signals that is broadly used, therefore they
can be received on existing single-frequency receivers thathave been modified. The concept of SLAS is standalone
DGNSS positioning like SBAS, and the concept image of
SLAS is as shown below (Fig. 6).

Fig. 6. Concept Image of SLAS

Fig. 7. Antenna (GrAnt-G5T) and Receivers (AQLOC and SPS855)

B. Kinematic Positioning Experiment
In the experiment on kinematic positioning, the data wascollected on an ordinal road in nearly open-sky condition, on ahighway (mainly in Metropolitan Expressway), and on the sea(Tokyo Bay). We adopted the RTK fixed solution of the
SPS855 receiver for the true positions, and the horizontal andheight errors were calculated by taking the difference of thedata from the true positions. The schematic of the setup
comprising various equipment used in the ordinal roadexperiment using a car is as shown below (Fig. 8).

Fig. 8. Settings of Equipment in the Experimental Car

This service is expected to be used mainly for purposes thatare not heavily affected by time lags such as pedestrians,
bicycles, and ships. This service can be used on L1S-signal
receivers.

The data acquired in the sea experiment conducted on January
16th, 2018 was also evaluated in the same way as the car
experiment. A photograph of the ship used in the experiment is
as shown below (Fig. 9).

III. CLAS EXPERIMENTExperiment Description
We conducted the experiments on static and kinematic
positioning using CLAS at the rooftop of the number 4
research building in Etchujima campus of Tokyo University ofMarine Science and Technology (TUMSAT), near the campus
in Tokyo, and in Chiba prefecture. We used an AQLOC
receiver manufactured by Mitsubishi Electric Corporation,
comprising a L6 signal band decoder and a CLAS RTK
positioning engine. As the experiments were conducted in
winter season and in a specific location as mentioned above,
the results may not apply to the country as a whole. The
correction information sent from the unit 1 of QZSS (PRN
number: 193) was used for the L6 signal band.
A. Static Positioning Experiment
In the experiment on static positioning, the antenna (GrAntG5T of JANAD GNSS make) was installed on the rooftop of
the number 4 research building, and the daily data (24 happroximately) was collected for the period from late
December 2017 to late January 2018. The horizontal and heighterrors (in time series) were calculated by taking the differenceof the data (AQLOC receiver) from the true positions (SPS855
receiver of Trimble Inc.). The photographs of the antenna andthe two receivers are as shown below (Fig. 7).

Fig. 9. Picture of the Experimental Ship (Shioji Maru)

C. Results
The conditions of the analysis and the statistical parameters(standard deviation, 95 percentile) used to analyze the results
are as shown below (Fig. 10).

Fig. 10. The Condition of Analysis and Analysis Items

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The specification of positioning accuracy of the CLAS service
and a map of the service area (made public by the cabinet
office formally) are as shown in the following page (TABLE I
and Fig. 11), respectively.
TABLE I.

POSITIONING ACCURACY OF CLAS

however, the overall result is considered to be stable. The
experimental data comprised 70024 time points (RTK fixedepochs) out of an opportunity of 86400 seconds (3600 s perhour x 24 h per day). The standard deviation and 95 percentile
of horizontal and height errors are as shown below. (TABLE
II).
TABLE II.

RESULT OF CLAS STATIC POSITIONING (JANUARY 7)

Moreover, as the above result is only for a day, the data fornine days are as shown below (TABLE III).
TABLE III.

Fig. 11. Service Area of CLAS (Yellow Area)(i.)

The Result of CLAS Static Positioning

The 24-h data (January 7th-8th, 2018) of the horizontal andheight errors of the static positioning of CLAS are as shown
below (Fig. 12).

RESULT OF CLAS STATIC POSITIONING (ALL DATA)

It is observed that the horizontal error occasionally exceeds the
specified accuracy, while all the data points of the height error
are within the specified accuracy, especially within 10 cm.
These results reveal that the positioning data obtained using the
AQLOC receiver are satisfactory; thus, the RTK Fix rate andthe correction information from unit 1 of QZSS are found to be
in order, thereby confirming that the performance of CLAS
with regard to static positioning is well demonstrated.
(ii.)

The Result of CLAS Kinematic Positioning

First, the results of the experiment on CLAS kinematic
positioning conducted in an ordinal road in Ariake city and theroute of the car are as shown below (Fig. 14, Fig. 15, andTABLE IV). The experiment was conducted on December 21st,
2017.

Fig. 12. Horizontal and Height Error of CLAS Static Positioning

The discontinuity in the data correspond to the period when theelevation angle of unit 1 of QZSS was less than approximately
10 degrees as shown below (Fig. 13). Evidently, the L6 signal
band could not be captured during this period.

Fig. 14. Route of the Car (CLAS Experiment in Ariake)
Fig. 13. Elevation of the unit 1 of QZSSEven in the time zone in which the correction information was
stably acquired, there are a few deviations of larger magnitude;408

highway. The pictures of the overpass are as shown below (Fig.
18).

Fig. 18. Pictures of Overpasses (CLAS Experiment in Highway)

Fig. 15. Horizontal Error (CLAS Experiment in Ariake)
TABLE IV.

Since CLAS is a system based on open sky and semi-open sky
conditions such as a highway, a cycle slip is likely to occur
while crossing the obstacles like an overpass. Considering theintended application of CLAS to automobiles as well, it isimportant to assess the time taken by CLAS in re-fixing the lostconnection. The number of overpasses and the time taken for
re-fixing the connection are as shown below (TABLE VI) for
the survey grade receiver (SPS855) and the receiver for CLAS
(AQLOC).

RESULT (CLAS EXPERIMENT IN ARIAKE)

It is observed that the 95 percentile values for both horizontal
and height errors are less than 15 cm.

TABLE VI.

TIME REQUIRED FOR RE-FIXING (IN HIGHWAY)

Second, the results of the experiment on CLAS kinematic
positioning conducted on a highway (Metropolitan
Expressway) and the route of the car are as shown below (Fig.
16, Fig. 17, and TABLE V). The experiment was conducted on
January 4th, 2018.
It is observed that a discontinuity in the time span (during
which the RTK fixed solution could not be reached) is mostly
attributable to a cycle slip when the car passed under the
overpasses, and that the time taken by AQLOC receiver torecover RTK positioning was found to be fairly less comparedwith the conventional RTK positioning value. The time taken
for re-fixing was 14 s for the big overpasses and 7 s for thesmall overpasses in this experiment.
Third, the results of the CLAS kinematic positioning on thesea (Tokyo Bay) and the route of the ship are as shown below(Fig. 19, Fig. 20, and TABLE VII). The experiment was
conducted on January 16th, 2018.

Fig. 16. Route of the Car (CLAS Experiment in Highway)

Fig. 17. Horizontal Error (CLAS Experiment in Highway)
TABLE V.

RESULT (CLAS EXPERIMENT IN HIGHWAY)
Fig. 19. Route of the Ship (CLAS Experiment on the Sea)

It is observed that the 95 percentile value for the horizontal andheight errors were within 14 cm and 15 cm, respectively.
We also investigated the time required in re-fixing thesignal connection that gets lost while crossing an overpass on a

Fig. 20. Horizontal Error (CLAS Experiment on the Sea)

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TABLE VII.

RESULT (CLAS EXPERIMENT ON THE SEA)

It is observed that the 95 percentile value for the horizontal
error was approximately 8 cm and that for the height error wasapproximately 9 cm on the sea. Owing to the open-sky
condition and the speed of the ship, the result was better thanthe car experiment.

Fig. 22. Settings of Equipment in the Experimental Car

IV. PPP EXPERIMENTExperiment Description
In September 2017, we conducted the static and kinematic
positioning using PPP at the rooftop of the number 4 researchbuilding in Etchujima campus of TUMSAT and near thecampus in Tokyo. We used the receiver manufactured by
Magellan Systems Japan Inc. comprising a L6 signal band
decoder and a PPP engine. The correction information sentfrom the unit 1 of QZSS was used for the L6 signal band.
A. Static Positioning Experiment
In the experiment of the static positioning, GrAnt-G5T
antenna was installed on the rooftop of the number 4 researchbuilding and data was collected starting from the evening of
September 4th, 2017 until the moment the unit 1 of QZSS
remained active (about 12 h). The antenna and receiver are asshown below (Fig. 21). An SPS855 receiver was used to derive
the true position of the antenna.

Fig. 23. Route of the Car (PPP Experiment)

C. Results(i.)
The Result of PPP Static Positioning
The results of PPP static positioning are as shown below(Fig. 24 and TABLE VIII).

Fig. 21. Antenna (GrAnt-G5T) and Receivers (SPS855 and Magellan)

B. Kinematic Positioning Experiment
In the experiment of the kinematic positioning, data was
obtained on the ordinal road in nearly open-sky condition near
the Harumi Wharf in Tokyo on September 6th, 2017. A GPS703-GGG antenna manufactured by NovAtel Inc. was installedon the roof of the car. The true value and the evaluated value
are found to be the same as those described for the kinematic
positioning of CLAS. The setup of the equipment in theexperimental car and the route of the car are as shown below(Fig. 22 – Fig. 23). The unit 1 of QZSS was located at a highelevation angle.

Fig. 24. Horizontal Error of PPP Static Positioning
TABLE VIII.

RESULT OF PPP STATIC POSITIONING

Fig. 24 shows that the positioning flag indicates the positioning
method used to output the solution: 1 means Single Point
Positioning (SPP) and 5 means PPP. The positioning flag
shifted from SPP to PPP within 1 min after switching on thereceiver and a convergence within 10 cm in the horizontal
direction was achieved in approximately 15 min. In fact, thebias shown in TABLE VIII occurred between the convergencevalue and the F3 solution (RTK fixed solution) of the
Geospatial Information Authority of Japan in each direction;
therefore, except for the bias, almost the same accuracy as
CLAS can be expected after the convergence. Besides, thereason why the positioning flag was switched from PPP to SPP

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at the end of the session was that the time period when theelevation angle of unit 1 of QZSS was less than approximately
10 degrees came.
(ii.)

The Result of PPP Kinematic Positioning

The results of PPP kinematic positioning are as shown in
the following page (Fig. 25).

Fig. 27. Location Information of Reference Points (Kanto Area)

In Okinawa area, we selected five reference points for every
baseline length: approximately 11 km (Naha), 52 km (Ginoza),
102 km (Kunigami), 175 km (Wadomari), and 312 km (Naze),
considering “Itoman” as the reference station. In Kanto area,
we selected five reference points for every baseline length:
approximately 20 km (Jo-hoku), 50 km (Iwase), 100 km(Moriya), 204 km (YugawaraA), and 300 km (OmaezakiA),considering “Hitachi” as the reference station. The postprocessed results of the DGNSS positioning for each region
and each reference point are as shown in TABLE IX – TABLE
X and Fig. 28 – Fig. 29. Furthermore, the receiver used by eachreference point is NetR9 manufactured by Trimble Corporation,and the antenna (TRM59800.80) is also of Trimble, for thelocations other than “Hitachi” and “Naze”. For “Hitachi”, theantenna was TPSCR.G5 manufactured by TOPCON Inc. For“Naze”, the receiver was DELTA-G3T manufactured by
JAVAD GNSS Inc., and the antenna was TPSCR.G5. Bothequipment are intended for surveying.

Fig. 25. Horizontal Error of PPP Kinematic Positioning

It is observed from the first part of Fig. 25 that the PPP was notinitially converging because the car was travelling in urban
areas. The car waited for 15 min to converge at the start point
as shown in Fig. 23 before it started moving. A good accuracy
within 10 cm was achieved in motion, considering the nearly
open-sky condition; however, the same bias as the static
positioning was observed. When the car entered a section
surrounded by many obstacles, the output of the Magellan
receiver immediately started deteriorating.

TABLE IX.

DGNSS POSITIONING RESULT IN OKINAWA AREATABLE X.

DGNSS POSITIONING RESULT IN KANTO AREA

V. EVALUATION OF SLAS
The static positioning experiment was conducted using
SLAS for each baseline length (of the order of tens to hundredsof km) using the electronic reference points of the Geospatial
Information Authority of Japan. The data of the Kanto area
(around Tokyo) and Okinawa area (southernmost island inJapan) were acquired for each baseline length, and the DGNSS
positioning results were examined using the post-processing
function of RTKLIB (ver.2.4.3 b29). Since the correction
information superimposed on the L1S signal band was also
from the data of the electronic reference points like the CLAS,
the post-processed result of DGNSS positioning using the dataof the electronic reference points can be regarded as almost thesame performance of the real-time DGNSS positioning via
QZSS. The period of data used for the analysis was 24 h from12:00 AM (UTC) on 15th July to 12:00 AM on 16th July, andthe location information of the reference points is as shown
below (Fig. 26 – Fig. 27).

Fig. 26. Location Information of Reference Points (Okinawa Area)

411

are observed to meet the RMS value of the specification
comfortably in Kanto area. The results of Okinawa area met the
specification up to 175 km; however, the accuracy of DGNSS
positioning deteriorated at the 312-km point, and the result did
not meet the specification. It is also conceivable that thereceiver and antenna used at “Naze” differed from otherreference points; however, the effect of the ionosphere on the
results must also be suitably considered because the data is
obtained by the survey grade receiver and antenna. The actualreference points used for SLAS correction information(probably hundreds of reference points) are not published;however, it was confirmed that the possibility of deteriorationin the positioning accuracy of SLAS in the southern island inJapan is larger compared with Kanto area.

Fig. 28. Errors of Horizontal and Height Direction (Okinawa Area)

VI. SUMMARY AND FUTURE WORK
Fig. 29. Errors of Horizontal and Height Direction (Kanto Area)

In this paper, we reported the performances of the newreinforcement functions of QZSS (CLAS, PPP, and SLAS)
evaluated by means of suitable positioning experiments.

The daily average values of the RTK fixed solution are
uploaded on the Web of the Geospatial Information Authorityof Japan on daily basis, using which the average (Avg.),
standard deviation (Std.), and root mean square (RMS) were
calculated for each reference point using the value in RTK
positioning calculation as the precise position. The satellite
system used for the positioning calculation comprised GPS,QZSS and Galileo. Fig. 28 and Fig. 29 show the graphicalrepresentations of TABLE IX and TABLE X, respectively. In
Kanto area, even with the extension of baseline length to 300
km, the deterioration in accuracy was only marginal comparedto the 20 km point. In contrast, the accuracy deteriorated in
Okinawa area, where the baseline was extended to 312 km. The
specification of positioning accuracy and service area of SLAS
made public by the cabinet office formally are as shown below(TABLE XI and Fig. 30). Field (1) and Field (2) in TABLE XI
support (1) and (2) in Fig. 30.
TABLE XI.

The experimental results of kinematic positioning using
CLAS showed relatively less accuracy compared to publishedspecifications; however, as the 95 percentile values were notperceptibly deteriorated, the overall result was considered to besatisfactory. In addition, even in the case of a cycle slip under
the overpasses, it showed satisfactory performance as theresults matched fairly with the normal RTK positioning, andthe RTK Fix rate was also significantly high. The results wereobtained only in Tokyo and Chiba prefecture; therefore, it is
proposed to continue the investigation in other areas of Japan.
The exact causes of the outliers will also be explored. Studies
will also be conducted for the improvement of accuracy using
other related factors such as Doppler velocity. The observations
as above are applicable to the case of PPP as well.
In the case of SLAS, the ionosphere was found to be a
critical factor as it varies with the geographical location in
Japan. It was also confirmed that the deterioration of the
precision differs when the baseline length extends. It isconceivable that the performance of SLAS (after official
commencement of its operation) will be the same as theobserved experimental results of the DGNSS positioning. As
the experiments in the present study were conducted during
summer, we propose to capture the seasonal influence of thesolar activity in the next one year.

POSITIONING ACCURACY OF SLASREFERENCES
[1]

[2]

[3]
[4]

[5]

Fig. 30. Service Area of SLAS[6]

The present data used for the analysis is subject to Field (1). It
is observed that both the horizontal and the vertical directions

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Authors
Author’s picture

So Takahashi
Mr. So Takahashi received his Bachelor of Engineering degree in Logistics and Information Engineeringfrom Tokyo University of Marine Science and Technology, Tokyo, Japan, in 2017. He is now a graduate
student at Tokyo University of Marine Science and Technology. His research interests are low-cost RTKGNSS receivers and new services of QZSS.

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